The present invention relates to a water-repellent metal oxide film, and a method of forming the film on a glass substrate. The film-forming method belongs to the sol-gel process. The water-repellent coating is suitable for application, for example, to vehicular and architectural window glasses which are to be exposed to rain water.
It is well known to coat a glass substrate surface with a hard and abrasion-resistant film of a metal oxide such as silicon oxide. The metal oxide film can be formed, for example, by the sol-gel process using a solution of an organic metal compound such as a metal alkoxide.
Hitherto, there have various proposals to form a metal oxide film on a glass substrate surface for the purpose of improving water-repellency of the glass substrate.
For example, JP A-60-231442 discloses a water-repellent film on a glass substrate. The chemical composition of the film is not the same throughout the film. In fact, the film contains a higher content of a condensation product of a first organic silicon compound on the side of the glass substrate and a higher content of a condensation product of a second organic silicon compound or a fluorine compound on the exposed side. The second organic silicon compound contains a higher content of carbon or a lower content of oxygen as compared with the first organic silicon compound. Examples of the first and second organic silicon compounds are tetramethylsilane and tetraethoxysilane, respectively. However, it is necessary to take a plasma condensation method to form the film on the glass substrate. Furthermore, the film tends to be too thick in thickness.
JP A-3-247537 discloses a method of forming water-repellent film on a glass substrate. In this method, a solution of polydialkylsiloxane in which fluorine atoms are substituted for not less than 5% of hydrogen atoms of alkyl groups is applied to the glass substrate. However, the thus formed film is insufficient in mechanical strength and abrasion resistance.
JP A-1-246160 discloses a method of forming a silicon oxide film on a glass substrate surface. A coating solution is made up of a silicon alkoxide as a main component. However, this film is not superior in water repellency.
JP A-3-75243 discloses a metal oxide film to be formed on a vehicular front windshield. The film has an amorphous oxide which has at least one of Zr, Ti, Hf, Sn, Ta and In, and at least one of B and Si. However, this film is also insufficient in water repellency.
There have been other proposals to form on a glass substrate a multilayered metal oxide film having first and second metal oxide layers. The first layer is adherent to the glass substrate, and the second layer is water-repellent. To increase adhesion between the first and second layers, there have been some conventional methods to make the first layer's surface minutely rough. One example is etching the first layer with hydrofluoric acid, fluorine nitrate or the like. However, this method has the following drawbacks.
Hydrofluoric acid and fluorine nitrate are very hazardous against human body. Therefore, these compounds must be handled very cautiously. This lowers the production efficiency. Furthermore, it is difficult to precisely control the thickness of the first layer.
Another example of the above conventional methods is making the first layer minutely rough by the thermal decomposition of an organic polymer added to a metal alkoxide solution. However, this method has the following drawback.
Micro-pits which make the first layer minutely rough tend to disappear by the densification of the base layer after baking at a temperature not lower than 400.degree. C. This tends to make the first layer relatively flat.